This invention relates to a method and associated apparatus for use with an auxiliary impulse commutated inverter circuit for controlling the firing delay interval between firing an inverter auxiliary thyristor to commutate a then-conductive inverter main thyristor and subsequently gating a then non-conductive inverter main thyristor into conduction as a function of inverter load current and source voltage.
In many industrial situations where only a fixed frequency voltage source is present, such as DC or low frequency AC, it is often necessary and even desirable to excite an AC load, such as a synchronous machine, with a voltage of different frequency than the source voltage. Voltage conversion is accomplished in such instances by utilizing an inverter coupled between the fixed frequency voltage source and the load. DC to AC or AC to AC voltage conversion is obtained during the "invert" mode of inverter operation while AC to DC conversion is obtained in the "rectify" inverter operation mode.
Inverter circuits commonly employed in voltage conversion applications typically include a pair of first and second main thyristors, coupled in series-aiding fashion across the fixed frequency source and which are connected at the thyristor junction to the load. Current is supplied to the load in a direction determined by the conduction of a corresponding main thyristor. Each main thyristor is commutated, that is to say, current conduction is extinguished, by reverse-biasing the thyristor. This is accomplished by gating or firing an appropriate one of a pair of auxiliary thyristors which couple a source of commutating voltage across the corresponding then-conductive main thyristor causing the same to be reverse biased and extinguished. Such an inverter circuit, appropriately denominated as an "auxilary impulse commutated inverter", is described and claimed in my U.S. Pat. No. 3,207,974 issued Sept. 21, 1965 and assigned to the assignee of the present invention. To the extent necessary for understanding of such inverter circuits, reference should be had to the above-identified patent.
During operation, the auxiliary impulse commutated inverter supplies current from a fixed frequency voltage source by alternately gating the first and second main thyristors into conduction. As will be appreciated by those skilled in the art, it is crucial to the operation of such inverter circuits that the corresponding auxiliary-thyristor be gated to commutate a then-conductive main thyristor at a preselected interval prior to rendering the remaining incoming or non-conductive main thyristor conductive. This is necessary to prevent the occurrence of a "shoot-through", when both the first and second main thyristors are simultaneously conductive, effectively placing a short circuit across the voltage source. To avoid the occurrence of a "shoot-through", the auxiliary thyristor is usually gated into conduction a fixed time interval prior to firing the remaining non-conductive main thyristor. This fixed time interval is typically selected by balancing such factors as power loss during inverter commutation intervals against the minimum time interval required to achieve main thyristor commutation.
With a fixed firing delay interval between firing an auxiliary thyristor and firing the remaining non-conductive main thyristor, increasing commutation losses inevitably result from increased inverter load current. However, by adjusting the inverter firing delay interval as a function of a dynamic circuit parameter such as inverter commutation current or source voltage, such commutation losses can be significantly reduced.
One such prior circuit for controlling the inverter thyristor firing delay interval as a function of a dynamic inverter circuit parameter is disclosed by E. A. Skogsholm et al. in U.S. Pat. No. 3,852,657, issued Dec. 3, 1974 and assigned to the assignee of the present invention. Skogsholm et al. sense inverter commutation (auxiliary inverter thyristor) current and render the incoming or remaining non-conductive thyristor conductive when the inverter commutation current amplitude decreases below a predetermined value.
Reduction of the inverter firing delay interval for a corresponding increase in inverter load current is accomplished in the apparatus of Skogsholm by increasing the commutation voltage applied across a then-conductive main inverter thyristor. The increased commutation voltage is obtained by "pumping up" or increasing the potential across a commutating capacitor. However, increasing the commutating capacitor voltage to reduce the inverter firing delay interval requires a large high-voltage commutating components, which components dissipate substantial power.
The method and associated apparatus, according to the present invention, control the inverter thyristor firing delay interval as a function of inverter load current and source voltage without dissipating substantial power.
It is an object of the present invention to provide a method and associated apparatus for varying the delay between firing an auxiliary thyristor and subsequently firing a nonconductive incoming main thyristor within an auxiliary impulse commutated inverter to reduce inverter commutation losses and prevent inverter interference.
It is another object of the present invention to provide a method and associated apparatus for varying the delay between firing an auxiliary thyristor and subsequently firing a nonconductive incoming main thyristor within an auxiliary impulse commutated inverter to limit inverter commutation time.